Nigerian extremists have issued direct threats to France in retaliation for military action in Mali, the French consulate here said Monday, while warning its citizens against travel to northern Nigeria.

"A growing situation of insecurity is resulting from the intervention undertaken in Mali against the advance of terrorist groups," the advisory from the consulate said. "In retaliation, Nigerian terrorist groups have issued direct threats against France and French citizens."

The nature of the threats was unclear, and a French official declined to elaborate when contacted by AFP. The statement added that "in these conditions, the risks of attack and especially kidnapping are very serious. The area of (extremist) action is the north, but we cannot rule out that it can happen anywhere," a French diplomat said on condition of anonymity. "That is why we are raising the alert level."

There has also been intense speculation over links between extremists in Nigeria and northern Mali. Members of Boko Haram are said to have trained with Al-Qaeda's north African branch in northern Mali, but it is unclear whether further ties have been developed.